A new law that allows for citizen's arrest is now in force but you better know what it allows you to do and what it doesn't, the police have warned.

New Zealanders making a citizen’s arrest can still face criminal charges or be sued if they get it wrong, police say in new guidance explaining what people must do when detaining someone.

The new law follows recommendations by the government's ministerial advisory group on retail crime headed by Sunny Kaushal. It came into force last week.

Previously, citizens' arrest powers only applied between 9pm and 6am or to crimes carrying at least three years' imprisonment. Not anymore.

“The law should protect law-abiding New Zealanders every hour of every day, not only at certain hours of the day. Offenders don’t check the clock before committing crime," says Kaushal, the chief architect of the citizen's arrest law.

Sunny Kaushal, chair of the erstwhile advisory group on victims of retail crime. (Supplied photo)

Police say a citizen’s arrest can be made when a person finds someone committing, or believes on reasonable and probable grounds that someone has committed, an offence under the Crimes Act 1961. That includes offences such as theft, assault and burglary.

But the police stress that making an arrest carries both legal and personal risks. In most situations, they say, the safest option is to call 111, observe what is happening and allow police to respond.

People should not intervene if doing so could put themselves or others in danger, including where a person may have a hidden weapon, the police say.

For anyone who does decide to make a citizen’s arrest, the first requirement is to be sure they have the correct person and have reasonable and probable grounds for believing a Crimes Act offence has been committed.

The person being detained must be clearly told that a citizen’s arrest is being made and that they are being held until police arrive. Police must then be called as soon as practicable, and anyone making the arrest must follow police directions, including releasing the detained person if instructed to do so.

Police say people should ask for the person’s cooperation and avoid using force. Force should only be used where it is necessary and proportionate, and even then only the minimum amount required.

What counts as reasonable force will depend on the circumstances. Using excessive force can result in criminal or civil legal action. Failure to meet the legal responsibilities associated with a citizen’s arrest could lead to an assault charge or the person making the arrest being sued.

There are also clear limits on what can happen once someone has been detained. Police say the person making the arrest must not question the detainee or search them or their belongings. They should also be prepared to give police a statement about what happened.

The person making the arrest becomes responsible for the safety of the detainee and those around them while they are being held.

Police say this includes considering whether the detained person needs medical help or may be vulnerable because of their age, disability or intoxication. Their physical position must also be monitored.

Police specifically warn people not to kneel or sit on a detained person, or put pressure on their neck or head, because of the danger of positional asphyxia or suffocation. The detainee’s welfare should be checked regularly.

Those making citizen’s arrests must act reasonably and proportionately at all times. Excessive force, an unlawful arrest or failing without reasonable excuse to follow police directions could expose them to criminal or civil proceedings.

The legal protection available also depends on the circumstances of the arrest.

Police say Section 35 of the Crimes Act provides that someone who finds another person committing an offence against the Act is “justified” in arresting them without a warrant. 

Under Section 36, a person who finds someone committing a Crimes Act offence by night and has reasonable and probable grounds for believing the offence is being committed is protected from criminal responsibility, but could still face civil proceedings.

Section 37 similarly provides protection from criminal responsibility where someone has reasonable and probable grounds to believe a Crimes Act offence has already been committed, while section 38 applies where a person believed to have committed an offence is escaping and being pursued by someone lawfully entitled to arrest them.